1. Technical Field
The present invention relates generally to photovoltaic devices containing a Group VI element. More particularly, this invention relates to the creation of thermodynamically stable, non-moving compounds within a photovoltaic device, for example having a cadmium telluride p-type layer with a metal-containing (e.g., copper-containing) electrical contact, upon exposure to fluorine atoms (e.g., via plasma treatment).
2. Description of Related Art
The need for affordable, renewable sources of energy has become more of a focus of public interest and scientific research in light of the rising price of traditional energy sources. Photovoltaic cells have the ability to convert sunlight, a practically infinite energy source, into direct electrical current. Single crystal photovoltaic cells exhibit high operation performance, up to 25-30% conversion efficiency, but are relatively costly to manufacture, and require a significant amount of raw materials. Polycrystalline and amorphous materials can also be used however they are relatively less efficient and less expensive.
A Group II-VI thin film photovoltaic cell comprised of, for example, polycrystalline cadmium telluride (CdTe) and cadmium sulfide (CdS), elements from Group II and VI of the Periodic Table of Elements, has shown great promise as research has successfully introduced the use of polymers, dye-sensitized materials, and thin films of either amorphous or polycrystalline materials. CdTe has a very high absorption coefficient of incident light due to its band gap matching well to that of sunlight and the CdS/CdTe photovoltaic cell has demonstrated conversion efficiencies of up to 16.5% in the laboratory.
There are many details and different methods of fabricating a CdTe solar cell; a description of one process follows. The physical structure of a typical CdS/CdTe photovoltaic cell typically starts with a glass superstrate and a transparent conducting oxide (TCO) layer, such as tin oxide, indium tin oxide, or cadmium stannate, placed on the superstrate. Then, for example, a CdS layer, doped to create a n-type material (i.e., the majority charge carriers within the layer are electrons), is deposited on to the TCO surface. The n-CdS layer has a wide band gap, wide enough to allow most incident solar radiation through thus serving as a transparent, or window, layer. The n-CdS layer is typically a thin layer—about 0.1 micrometer (μm) and it allows more of the incident sunlight to pass through the layer.
Another layer, a p-type CdTe layer (i.e., the majority charge carriers within this layer are holes), is deposited atop the CdS layer. The current industry standard for creating a good quality p-type CdTe layer is to anneal, usually at about 400° C., the CdTe layer in the presence of CdCl2. The p-CdTe layer, typically much thicker than the CdS layer at about 2 to 10 μm thick, absorbs the majority of incident light due to its previously-described high absorption coefficient. The bulk of electrons and holes freed by the absorption of photons are created within this layer.
A charge imbalance across the boundary between the n-CdS layer and the p-CdTe layer, or the p-n junction, creates an electric field. This electric field, spanning the p-n junction, separates the freed electrons and holes. Holes may travel from the p-type side across the junction to the n-type side and, electrons may pass from the n-type side to the p-type side. However, the p-n junction allows conventional current to flow in only one direction thus behaving as a diode.
Electrical contacts on the photovoltaic cell allow current to be collected and flow to external circuits. In the CdS/CdTe photovoltaic cell, the transparent conducting oxide layer serves as the front contact. A metallic layer placed onto the p-CdTe layer, functions as the back, or rear contact. However, a stable, low-resistance ohmic contact with p-CdTe is very difficult to obtain due to CdTe's band gap and high electron affinity. Together, for an ohmic contact to p-CdTe, these characteristics would require the back contact material to have a very high work function—greater than 5.78 eV—in order to facilitate hole transport from CdTe. Unfortunately, no metal satisfies this requirement. In addition to the difficulty p-CdTe itself poses to the formation of an ohmic back contact, the metal contact-CdTe interface itself can also create a Schottky barrier of sufficient height and width to significantly limit hole transport from CdTe. Essentially, this barrier behaves as a diode with characteristics directly opposite to that of the p-n junction. Thus, the Schottky barrier diode blocks the desired flow of photo-generated charge carriers, greatly increasing the resistance of the back contact and limiting the total cell current generated, which ultimately reduces the efficiency of the solar cell.
Currently, the art addresses creating a Te-rich surface in order to obtain a low-resistance electrical contact to p-type CdTe and the most successful contacts on p-CdTe first create, by various methods, a Te-rich thin layer at the p-CdTe surface, then the method calls for applying a metal, which often is copper. Copper is believed to be effective because it increases the effective doping level by creating a p+-layer near the p-CdTe surface at the electrical contact through interdiffusion with CdTe assisting in the establishment of quantum mechanical tunneling that simulates an ohmic contact. This interdiffusion can also precipitate the formation of a physically distinct layer of cuprous telluride (Cu2Te). The Cu2Te layer is thought to be important in achieving an ohmic contact to p-CdTe.
Copper has another more dubious distinction; its diffusion is known to be one of the main causes of instability and degradation of the performance CdTe photovoltaic cells. Copper is a fast diffuser that moves within the crystal and along the grain boundaries of the characteristically poly-crystalline thin-film CdTe. It tends to diffuse away from the contact, interact with other constituents of the cell and accumulate at the p-n junction and in the CdS layer. This Cu diffusion causes the back contact to degrade, making the contact more resistive, less able to transport charge carriers, and its accumulation decreases cell performance and stability. Although copper can react with tellurium to form Cu2Te, thought to be a beneficial compound, it is believed that this compound is less thermodynamically favorable than CdTe. Further, the conversion of Cu2Te to cupric telluride (CuTe), which is a less thermodynamically stable, is also believed to contribute to cell degradation. Tellurium, being more desirous to bind with the cadmium, leaves some quantity of Cu2Te and CuTe thereby allowing copper to diffuse.
Research continues to address the problem of copper diffusion away from, and subsequent degradation of, the back contact. Some research has contemplated eliminating the copper back contact all together through heavily doping the CdTe surface near the back contact with p-dopants such as mercury, silver, and antimony, or the use of copper-free contacts such as antimony telluride (Sb2Te3). Other research has investigated forming an interfacial layer (IFL), using aniline, between the p-CdTe and a contact metal of Cu or Au. However, there currently is no definitive solution that provides an efficient method to create a stable low-resistance back contact in CdTe photovoltaic cells.